This invention relates generally to satellite-based communication systems, and more particularly to a ground terminal design applicable to satellite-based communication systems having multiple satellites.
Satellite-based communication systems which provide wireless access between separated user terminals on the earth are designed to give users of laptop PCs, handheld computers, and other similar devices very high-speed two-way data links with the Internet, their corporate Intranets, each other, and any other desired network. The service coverage is international, and utilizes high-power satellite transponders that route data from one user terminal to another.
The satellite-based systems presently under consideration include at least two different types of satellite constellations. One type, such as that described as the Teledesic(trademark) or ICO Global systems, provides a plurality of satellites in low-earth or medium-earth orbits, in which satellites enter and exit from view of a user terminal and require handoffs from one satellite to another as the individual satellites traverse into and out of view of a particular user. These handoffs typically occur via intersatellite crosslinks which route incoming data from a user from one satellite to the next. These intersatellite crosslinks are expensive to manufacture, are heavy and thus increase launch costs, take up valuable real estate on the satellite, and consume precious power.
A second type of system, such as that known as Astrolink(trademark), uses satellites in geostationary or geosynchronous orbit (GEO), which essentially are viewed as stationary in the sky by a user on the earth. Satellite handoffs are not necessary in the basic system, since only one satellite is ever in the field of view of the user.
However, in order to provide redundancy and to provide increased capacity, certain GEO systems may utilize two or more satellites in one node along the geostationary arc. This use of multiple satellites at one node can cause a problem, in that the ground terminals transmit and receive to and from only one satellite at a time. Communications between users accessing different satellites must be routed either on inter-satellite crosslinks or through a ground-bounce gateway so that user data is routed to the satellite to which the user is connected. Users within a particular geographic region will be divided between the two satellites in the node. As a result, statistically, approximately half the data within a region could be routed between satellites. However, the ground-bounce gateway is an inefficient utilization of system resources. It therefore is desirable to develop user ground terminals which can be used in a satellite-based communication system involving multiple satellites without requiring either intersatellite crosslinks or a double hop through a ground gateway.
In accordance with the present invention, the aforementioned need in the prior art is met by a communications system which includes two or more satellites each including means for receiving RF signals through an uplink channel and for transmitting RF signals through a downlink channel distinct from said first uplink channel. The system is made feasible by the use of user terminals, each able to transmit RF signals to only one of the satellites, but having the capability to receive RF signals from at least two of the satellites.